The present invention relates to a mobile data carrier more especially in disk form comprising pre-etchings giving material form to tracks along which optically readable data may be recorded. The invention also relates to a device for radially following these tracks.
Numerous radial tracking processes have been described in the prior art. When it is a question of recording data in sequential form, for example video data, generally the tracks are not materialized beforehand but rather created in real time at the time of the recording. The data is recorded along tracks having the shape of a single spiral extending from the peripheral zone of the disk to a central zone or reversely, or else having the form of concentric circles centered on the axis of rotation of the disk.
In the simplest systems, the accuracy of the members causing the recording head to advance is used for creating this track. During reading, data recorded for example in the form of micro-reliefs, interferes with a reading beam focused in the plane of the recording face of the disk. The movement of these micro-reliefs under the focusing spot modulates the beam and this modulation is detected by means of photoelectric cells converting the light intensity variations into electric signals. These electric signals may also be used for tracking.
The process which has just been described requires the advance of the recording head to have a very high mechanical stability, so as to avoid two successive grooves from overlapping or at least being poorly discernable during reading. For improving the system which has just been described, there has been proposed in patent US-A-No. 4 275 275 a process using the last track recorded as reference. With the system for mechanically advancing the recording head is associated a system for optical deflection of the recording spot. The reading spot is forced to follow a groove already recorded on the track by conventional positional control. By means of the optical deflection system, the recording spot is maintained at a constant distance, equal to a whole number multiple of the pitch of the track, from the reading spot.
However, when it is desired to record data in a random fashion, for example in data processing applications, it is no longer possible to use the processes which have just been described or similar processes. It is in general necessary to give beforehand material form to the tracks in front of which data may be recorded. For this, it is usual to create a pre-etching in any form. In one embodiment described in patents U.S. Pat. No.-A-4 252 889, U.S. Pat. No.-A-4 288 510 and U.S. Pat. No.-A-4 334 007, during manufacture of the data carrier, the tracks are materialized in the form of a smooth groove created in an auxiliary layer of this carrier. These tracks may be detected even in the absence of any data recording, this recording being effected in a subsequent phase in a photosensitive or thermo-sensitive layer in contact with the auxiliary layer.
In a preferred variation of the prior art, the pre-etched tracks may merge with the regions in which the data is recorded. Then, a so-called monotrack system is formed.
In other processes, the pre-etched track or tracks are distinct from the tracks along which the data is recorded. Then so-called dual track or multi-track systems are formed. To discriminate these two types of track the pre-etching may consist of a signal which may be broken down into a first frequency spectrum and the data which may be broken down into a second separate frequency spectrum. During writing, the reading beam may be used for following the pre-etched track as in the second above-mentioned American patent application.
The main drawback of the process which has just been described is that it does not allow maximum recording density since it requires, at the minimum, an additional pre-etched track for a pre-etched data track. Furthermore, it requires the use of two beams, one for radially following the etched track and the other for writing in or reading data from the track intended for the recording.
Data carriers of the mono-track type, comprising a pre-etching, are not free from disadvantages either. They generally require the use of two beams, one for recording and the other for radial tracking. Furthermore, though the pre-etched track is easily discernable from the rest of the disk (intertrack zones) in the absence of data recording, it is no longer the case when data is recorded. The result may be, if precautions are not taken, contrast reversals leading to tracking errors.
The present invention provides a mobile data carrier structure with pre-etching in the form of a disk obviating the drawbacks of the prior art and compatible with monotrack -monobeam systems.